U.S. patent number 7,630,109 [Application Number 11/424,033] was granted by the patent office on 2009-12-08 for covert security coating.
This patent grant is currently assigned to JDS Uniphase Corporation. Invention is credited to Roy Bie, Roger W. Phillips.
United States Patent |
7,630,109 |
Phillips , et al. |
December 8, 2009 |
**Please see images for:
( Certificate of Correction ) ** |
Covert security coating
Abstract
A multilayer thin film filter is disclosed an organic dielectric
layer serving as a spacer layer in a Fabry-Perot structure. The
dielectric has embossed regions of varying thicknesses wherein the
thickness within a region is substantially uniform. Each different
region of a different thickness produces a different color (shift).
The size of one of the embossed adjacent regions is such that the
color of said one region is uniform and cannot be seen by a human
eye as different in color from the uniform color of an adjacent
region thereto, and wherein the color within a region can be seen
with magnification of at least 10:1. This serves as a covert color
coding system useful as a security device.
Inventors: |
Phillips; Roger W. (Santa Rosa,
CA), Bie; Roy (Rohnert Park, CA) |
Assignee: |
JDS Uniphase Corporation
(Milpitas, CA)
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Family
ID: |
37478783 |
Appl.
No.: |
11/424,033 |
Filed: |
June 14, 2006 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20060285184 A1 |
Dec 21, 2006 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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60691499 |
Jun 17, 2005 |
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Current U.S.
Class: |
359/2; 359/567;
283/91 |
Current CPC
Class: |
B41M
3/14 (20130101); G02B 5/287 (20130101); G02B
5/284 (20130101); B42D 25/425 (20141001); G03H
1/02 (20130101); B42D 25/328 (20141001); G03H
1/0236 (20130101); G03H 1/028 (20130101); G03H
1/0011 (20130101); G03H 2270/24 (20130101); B42D
2035/24 (20130101); G03H 2225/31 (20130101); G03H
2001/188 (20130101); G03H 1/0244 (20130101); G03H
1/0256 (20130101); B41M 3/003 (20130101) |
Current International
Class: |
G03H
1/00 (20060101); G02B 5/18 (20060101) |
Field of
Search: |
;359/2,567
;283/85,87,91 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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43 43 387 |
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Jun 1995 |
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DE |
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0 756 945 |
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Feb 1997 |
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EP |
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1 353 197 |
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Oct 2003 |
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EP |
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1 741 757 |
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Jan 2007 |
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EP |
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WO 98/12583 |
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Mar 1998 |
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WO |
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WO 00/08596 |
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Feb 2000 |
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WO |
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WO 02/00446 |
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Jan 2002 |
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WO |
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WO 2005/017048 |
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Feb 2005 |
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WO |
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2005/038136 |
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Apr 2005 |
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WO |
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Other References
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Dallas TX, Apr. 24-29, 2004, pp. 6. cited by other .
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Prepared by the Sol-Gel Technique", Journal of Inorganic and
Organometallic Polymers, vol. 1, No. 1, 1991, pp. 87-103. cited by
other .
Jeffrey I. Zink, et al., "Optical Probes and Properties of
Aluminosilicate Glasses Prepared By The Sol-Gel-Method", Polym.
Mater. Sci. Eng., pp. 204-208 (1989). cited by other .
Don W. Tomkins, Kurz Hastings, Transparent overlays for security
printing and plastic ID cards, pp. 1-8, Nov. 1997. cited by other
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`Optical Variable Devices` (OVD's) For Banknotes, Security
Documents and Plastic Cards," San Diego, Apr. 1-3, 1987. cited by
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SPIE, vol. 1210 Optical Security and Anticounterfeiting Systems,
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Products," SPIE, vol. 2659, Jun. 1996, pp. 10-20. cited by other
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Steve McGrew, "Countermeasures Against Hologram Counterfeiting,"
Internet site www.iea.com/-nli/publications/countermeasures.htm,
Jan. 6, 2000. cited by other .
Roger W. Phillips, "Optically Variable Films, Pigments, and Inks,"
SPIE vol. 1323, Optical Thin Films III: New Developments, 1990, pp.
98-109. cited by other .
Roger W. Phillips and Anton F. Bleikolm, "Optical Coatings for
Document Security," Applied Optics, vol. 35, No. 28, Oct. 1, 1996,
pp. 5529-5534. cited by other .
J.A. Dobrowolski; F.C. Ho; and, A. Waldorf, "Research on Thin Film
Anticounterfeiting Coatings at the National Research Council of
Canada," Applied Optics, vol. 28, No. 15, Jul. 15, 1989, pp.
2702-2717. cited by other .
J. Rolfe, "Optically Variable Devices for Use on Bank Notes," SPIE,
vol. 1210 Optical Security and Anticounterfeiting Systems, pp.
14-19, 1990. cited by other .
OVD Kinegram Cor, "OVD Kinegram Management of Light to Provide
Security," Internet site www.kiknegram.com/xhome/home.html, Dec.
17, 1999. cited by other .
I.M. Boswarva, et al., "Roll Coater System for the Production of
Optically Variable Devices (OVD's) for Security Applications,"
Proceedings, 33rd Annual Technical Conference, Society of Vacuum
Coaters. pp. 103-109 (1990). cited by other.
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Primary Examiner: Amari; Alessandro
Attorney, Agent or Firm: Pequignot; Matthew A. Pequignot +
Myers LLC
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
The present invention claims priority from U.S. Patent Application
No. 60/691,499 filed Jun. 17, 2005, which is incorporated herein by
reference for all purposes.
Claims
What is claimed is:
1. A multilayer thin film filter having an organic dielectric layer
therein, spanning a plurality of regions of the filter, wherein the
dielectric layer is embossed to define the plurality of regions of
different uniform thicknesses, wherein some adjacent regions of the
dielectric layer have a different uniform thickness, and wherein
the size of one of the embossed adjacent regions is such that the
color of said one region is uniform and cannot be seen by a human
eye as different in color from the uniform color of an adjacent
region thereto, and wherein the color within a region can be seen
with magnification of at least 10:1.
2. A multilayer thin film filter as defined in claim 1, wherein one
of the multi layers is an absorber layer that is more uniform in
thickness than the embossed organic dielectric layer.
3. A multilayer thin film filter as defined in claim 2, wherein one
of the multi layers is a reflective layer that is more uniform in
thickness than the embossed organic dielectric layer.
4. A multilayer thin film filter as defined in claim 3, wherein the
reflector layer is more uniform in thickness than the absorber
layer.
5. A multilayer thin film filter as defined in claim 3 further
comprising a second embossed organic dielectric layer on an
opposite side of the reflective layer and a second absorber layer
over the second embossed organic dielectric layer.
6. A multilayer thin film filter as defined in claim 5 wherein the
filter is symmetrical and wherein the reflective layer is
sandwiched between the organic dielectric layer and the second
organic dielectric layer and where the absorber layer and the
second absorber layer cover the embossed dielectric layer and the
second embossed dielectric layer respectively.
7. A multilayer thin film filter as defined in claim 1, wherein a
color difference can be seen with a magnification of at least 100
times, and wherein a color difference is imperceptible with a
magnification of 10 times or less.
8. A multilayer thin film filter as defined in claim 1, wherein a
region of the embossed dielectric layer having a different
thickness than another adjacent region is embossed with a
diffraction grating therein, and wherein the average thickness of
said embossed region is different than the thickness of an adjacent
region.
9. A multilayer thin film filter as defined in claim 8, wherein the
embossed region provides diffractive effect and color shifting
effects, and wherein the adjacent region only provides color
shifting effects.
10. A multilayer thin film filter as defined in claim 9, wherein
the thin film filter is a chromagram.
11. A multilayer thin film filter as defined in claim 1, wherein
the filter comprises a pigment flake, and wherein one of the
embossed regions provides a covert security feature.
12. A multilayer thin film filter as defined in claim 11 wherein
the covert security feature is in the form of indicia.
13. A multilayer thin film filter as defined in claim 1 wherein the
plurality of different color regions define a coven color code that
is only discernable with magnification.
14. A multilayer thin film filter as defined in claim 1, wherein a
region of the embossed dielectric layer has non-sinusoidal
impressions in adjacent regions and wherein the impressions form
cavities with flat bottoms and wherein the cavities are below eye
resolution.
15. A multilayer thin film filter as defined in claim 14, wherein
cavities are square, rectangular, circular or triangular.
16. A multilayer thin film filter as defined in claim 1, wherein
the plurality of different color regions differ in their color from
one another by at least a .DELTA.E value of 10.
17. A multilayer thin film filter having an organic dielectric
layer therein sandwiched between an absorber and reflector layer,
wherein the dielectric layer is embossed to provide a coven
security information only discernible with magnification wherein
the absorber layer and the reflector layer are non-embossed and
have substantially a uniform thickness.
18. A multilayer thin film filter comprising: an organic dielectric
layer therein, spanning a plurality of regions of the filter,
wherein the dielectric layer is embossed in at least one region to
define a different thickness than in an adjacent region and wherein
the embossing is of a dimension that produces an optical effect
that cannot be seen by a human eye without magnification of at
least 10 times; an absorber layer covering the organic dielectric
layer; and, a reflector layer supporting the organic dielectric
layer.
Description
FIELD OF THE INVENTION
This invention relates generally to thin film optical coatings for
use in producing security articles and to the production of
diffractive surfaces such as holograms or gratings having color
shifting or optically variable backgrounds which can be used as
security articles in a variety of applications. More particularly
this invention relates to the field of coating and or stamping a
dielectric substrate to provide a grating or hologram preferably
within a vacuum roll coating chamber while in a vacuum to produce a
Chromagram.TM. type of device or to produce a base device on which
to fabricate a Chromagram.TM. type of device. The invention also
relates to the manufacture of a covert optical device having a
dielectric layer of varying thickness. By way of example a
Chromagram may have a light transmissive substrate having a
diffraction grating or hologram etched or embossed into the
substrate and wherein patterning of some form is done on the
substrate, or the hologram or diffraction grating, generally in the
form of an opaque reflective coating. The remaining windows or
regions absent the reflective coating can be uncoated or may have
another coating covering the windows that is visually distinct from
the opaque reflective patterned coating. For example color shifting
coatings may be used adjacent to a highly reflective aluminum
pattern.
BACKGROUND OF THE INVENTION
The Relevant Technology
Security devices are being used more and more to protect currency
and other valuable documents such as passports, drivers' licenses,
green cards, identity cards and the like. These security devices
are also used to protect commercial products such as
pharmaceuticals, cosmetics, cigarettes, liquor, electronic media,
wearing apparel, toys and spare parts for automobiles and aircraft
from counterfeiting. In fact, it is estimated that counterfeit
articles now comprise between 5% and 7% of world trade. Holograms
attached to such articles have been the traditional method to foil
counterfeiters.
Color shifting pigments and colorants have been used in numerous
applications, ranging from automobile paints to anti-counterfeiting
inks for security documents and currency. Such pigments and
colorants exhibit the property of changing color upon variation of
the angle of incident light, or as the viewing angle of the
observer is shifted. The primary method used to achieve such color
shifting colorants is to disperse small flakes, which are typically
composed of multiple layers of thin films having particular optical
characteristics, throughout a medium such as paint or ink that may
then be subsequently applied to the surface of an object.
Diffraction patterns and embossments, and the related field of
holographs, have begun to find wide-ranging practical applications
due to their aesthetic and utilitarian visual effects. One very
desirable decorative effect is the iridescent visual effect created
by a diffraction grating. This striking visual effect occurs when
ambient light is diffracted into its color components by reflection
from the diffraction grating. In general, diffraction gratings are
essentially repetitive structures made of lines or grooves in a
material to form a peak and trough structure. Desired optical
effects within the visible spectrum occur when diffraction gratings
have regularly spaced grooves in the range of hundreds to thousands
of lines per millimeter on a reflective surface.
Diffraction grating technology has been employed in the formation
of two-dimensional holographic patterns which create the illusion
of a three-dimensional image to an observer. Three-dimensional
holograms have also been developed based on differences in
refractive indices in a polymer using crossed laser beams,
including one reference beam and one object beam. Such holograms
are called volume holograms or 3D holograms. Furthermore, the use
of holographic images on various objects to discourage
counterfeiting has found widespread application.
There currently exist several applications for surfaces embossed
with holographic patterns which range from decorative packaging
such as gift wrap, to security documents such as bank notes and
credit cards. Two-dimensional holograms typically utilize
diffraction patterns, which have been formed on a plastic surface.
In some cases, a holographic image which has been embossed on such
a surface can be visible without further processing; however, it is
generally necessary, in order to achieve maximum optical effects,
to place a reflective layer, typically a thin metal layer such as
aluminum, or a high index layer, like ZnS, onto the embossed
surface. The reflective layer substantially increases the
visibility of the diffraction pattern embossment.
Every type of first order diffraction structure, including
conventional holograms and grating images, has a major shortcoming
even if encapsulated in a rigid plastic. When diffuse light
sources, such as ordinary room lights or an overcast sky, are used
to illuminate the holographic image, all diffraction orders expand
and overlap so that the diffraction colors are lost and not much of
the visual information contained in the hologram is revealed. What
is typically seen is only a silver colored reflection from the
embossed surface and all such devices look silvery or pastel, at
best, under such viewing conditions. Thus, holographic images
generally require direct specular illumination in order to be
visualized. This means that for best viewing results, the
illuminating light must be incident at the same angle as the
viewing angle. In addition upon rotation by 90 degrees even in
specular light, the standard hologram disappears and all one sees
is a silver like patch since now the groves of the diffraction
pattern are mainly oriented in line with the incoming light as ones
eye; i.e. no diffraction occurs.
Since the use of security holograms has found widespread
application, there exists a substantial incentive for
counterfeiters to reproduce holograms, which are frequently used in
credit cards, banknotes, and the like. Thus, a hurdle that security
holograms must overcome to be truly secure, is the ease at which
such holograms can be counterfeited. One step and two step optical
copying, direct mechanical copying and even re-origination have
been extensively discussed over the Internet. Various ways to
counteract these methods have been explored but none of the
countermeasures, taken alone, has been found to be an effective
deterrent.
One of the methods used to reproduce holograms is to scan a laser
beam across the embossed surface and optically record the reflected
beam on a layer of a material such as a photo-polymerizable
polymer. The original pattern can subsequently be reproduced as a
counterfeit. Another method is to remove the protective covering
material from the embossed metal surface by ion etching, and then
when the embossed metal surface is exposed, a layer of metal such
as silver (or any other easily releasable layer) can be deposited.
This is followed by deposition of a layer of nickel, which is
subsequently released to form a counterfeiting embossing shim.
Due to the level of sophistication of counterfeiting methods, it
has become necessary to develop more advanced security measures.
One approach, disclosed in U.S. Pat. Nos. 5,624,076 and 5,672,410
to Miekka et al., embossed metal particles or optical stack flakes
are used to produce a holographic image pattern.
A further problem with security holograms is that it is difficult
for most people to identify and recollect the respective images
produced by such holograms for verification purposes. The ability
of the average person to authenticate a security hologram
conclusively is compromised by the complexity of its features and
by confusion with decorative diffractive packaging. Thus, most
people tend to confirm the presence of such a security device
rather than verifying the actual image. This provides the
opportunity for the use of poor counterfeits or the substitution of
commercial holograms for the genuine security hologram.
In other efforts to thwart counterfeiters, the hologram industry
has resorted to more complex images such as producing multiple
images as the security device is tilted to the right or left. These
enhanced images provide the observer with a high level of "flash"
or aesthetic appeal. Unfortunately, this added complexity does not
confer added security because this complex imagery is hard to
communicate and recollection of such imagery is difficult, if not
impossible, to remember.
U.S. Pat. No. 6,761,959 to Phillips et al, assigned to JDS Uniphase
Corp. discloses a security article having Chromagram.TM. thereon.
The chromagram provides both color shifting and holographic effects
to the viewer. In the '959 patent an organic substrate stamped with
a holographic grating or pattern is coated with a color shifting
multilayer film.
United States patent application 2005/0128543 in the name of
Phillips et al, assigned to JDS Uniphase Corp. discloses a more
complex type of Chromagram.TM. wherein patterning is shown. In some
regions, holographic effects are shown, and in other regions only
color shifting effects are visible.
Another United States patent application which discloses
diffraction gratings with color shifting coatings but deviates from
the teaching of Phillips et al, is U.S. patent application
2003/0058491, in the name of Holmes et al. United States Patent
application '491 appears to deviate from the teaching of Phillips
in that a decoupling layer is taught as way in which to separate
the diffraction grating effects from the color shifting effects.
Holmes suggests placing a decoupling layer between the relief
structure and the thin film reflection filter, which is described
to be a thin film reflection filter.
In contrast to Holmes et al. prior art U.S. Pat. No. 6,987,590
teaches a different novel Chromagram.TM. wherein a decoupling layer
is not required, but wherein separate color shifting and
holographic effects are exhibited. For example in FIG. 6 a color
shifting ink provides color shifting effects and a reflective
coating adjacent thereto provides holographic effects.
In all of these aforementioned security structures, coating is
suggested in common, known ways. That is, by first stamping a
grating, and subsequently applying the coating layers required to
create the desired patterns of reflective and color shifting
coatings. Although these prior art fabrication methods appear to
perform their intended function, of making Chromagram-like
structures having both holographic and color shifting effects, it
would be most advantageous, if the entire process or most of the
fabrication process was performed in-situ, within the vacuum roll
coating machine.
It would therefore be of substantial advantage to develop a
suitable, practicable, process and apparatus that would allow a
diffraction grating or hologram to be formed within a vacuum
coating chamber on an organic dielectric layer (ODL) wherein
coating of the hologram or grating with a reflective and or color
shifting coating was performed to the ODL before or after forming
the diffraction grating or hologram within the vacuum chamber
without breaking the vacuum.
More particularly, it is an object of this invention to provide an
in-line process for providing a hologram or grating in a roll type
process within a vacuum chamber without breaking vacuum.
Another aspect of this invention which can be fabricated in an
in-line system as mentioned above, or is not restricted to
manufacture in an in-line system is related to providing an organic
dielectric layer within a Fabry-Perot structure or a dielectric
stack formed structure, wherein the organic dielectric layer has a
varying thickness, and wherein the effects of the dielectric
structure of varying thickness can only be seen under
magnification.
The provision of a dielectric layer with a varying thickness has
been disclosed in U.S. Pat. No. 5,877,895 issued in the name of
Shaw et al. Mar. 2, 1999. Shaw et al disclose applying heat
variably to create a dielectric layer of varying thickness. Due to
the size of the apparatus necessary to provide a thickness
difference, adjacent regions of varying thickness are quite large
and the effects are noticeable, as is apparently desired.
There is no mention or suggestion with the Shaw et al. patent to
providing covert security features.
In contrast, in an aspect of this invention, a security device is
provided wherein a dielectric layer therein has a plurality of
adjacent regions. At least one adjacent region of the dielectric
layer has a thickness that is less than an adjacent region of the
same layer. The dimensions of at least one of the regions is small
enough such that a visual effect from the difference in the two
adjacent regions is not visible to the human eye, however a visual
color difference is visible with magnification of 10:1 or greater.
Preferably the different color regions differ in their color from
one another by at least a delta E value of 10.
It is a further object of the invention to provide a dot matrix
grating having a dielectric spacer layer having different
thicknesses throughout, so as to provide different visual colors in
accordance with the different thicknesses.
Thus, it is an object of the invention to provide a security device
having a dielectric layer forming a Fabry Perot cavity or within a
dielectric stack of dielectric layers wherein the dielectric layer
has varying thicknesses so as to form optical cavities exhibiting
different colors as visible light is incident thereon; and wherein
the visual effect of the different colors is not seen without
magnification. In addition, these different regions each have their
own color shift with viewing angle.
STATEMENT OF THE INVENTION
In accordance with an aspect of this invention, there is provided,
a multilayer thin film filter having an organic dielectric layer
therein, spanning a plurality of regions of the filter, wherein the
dielectric layer is embossed to define the plurality of regions of
different uniform thicknesses, wherein some adjacent regions of the
dielectric layer have a different uniform thickness, and wherein
the size of one of the embossed adjacent regions is such that the
color of said one region is uniform and cannot be seen by a human
eye as different in color from the uniform color of an adjacent
region thereto, and wherein the color within a region can be seen
with magnification of at least 10:1
In accordance with the invention, there is further provided, a
multilayer thin film filter having an organic dielectric layer
therein sandwiched between an absorber and reflector layer, wherein
the dielectric layer is embossed to provide a covert security
information only discernible with magnification.
In accordance with the invention, there is further provided, a
multilayer thin film filter comprising an organic dielectric layer
therein, spanning a plurality of regions of the filter, wherein the
dielectric layer is embossed in at least one region to define a
different thickness than in an adjacent region and wherein the
embossing is of a dimension that produces an optical effect that
cannot be seen by a human eye without magnification of at least 10
times; an absorber layer covering the organic dielectric layer;
and, a reflector layer supporting the organic dielectric layer.
In accordance with another aspect of the invention there is
provided, a multilayer thin film filter having a dielectric layer
having a first region embossed with a diffraction grating and
having adjacent regions that are absent a diffraction grating to
provide contrast, wherein both regions provide different color
shift effects when the filter is tilted with respect to the viewing
angle and wherein the embossed region provides diffractive and thin
film interference effects.
In accordance with another aspect of the invention, there is
provided, a method of coating comprising the steps of:
disposing within a vacuum chamber roll of light transmissive
substrate embossed with a diffraction grating or hologram; and,
patterning a reflector with an oil printing technique so as to
evaporate a reflective material within the vacuum chamber without
breaking vacuum.
In accordance with the invention there is provided a method of
providing a chromagram comprising the steps of:
embossing an organic coating;
curing the organic coating;
demetallized patterning of a reflective layer upon the embossed
organic coating;
curing through the substrate to fully cure the organic coating,
thereby allowing a relatively soft organic dielectric into which
the embossing can occur.
In accordance with the invention there is further provided a method
of coating is provided comprising the steps of:
disposing a releasable substrate roll of dielectric material into a
vacuum chamber;
embossing the dielectric material while within the vacuum chamber;
and,
coating the dielectric material while within the vacuum chamber;
wherein coating the dielectric material may be done before
embossing.
In accordance with the invention a method is provided for coating a
substrate comprising the steps of:
disposing the substrate within a vacuum roll coater;
embossing the substrate;
evaporating an absorber upon the substrate
depositing an organic layer upon the absorber layer;
patterning a reflector with upon the organic layer within the
vacuum roll coater;
performing steps (b) through (e) without breaking vacuum.
In accordance with another aspect of the invention a filter is
provided having an organic dielectric layer (ODL) which forms an
active part of the filter, wherein the ODL has varying thicknesses
and is sandwiched between an absorber layer and a reflector layer,
or wherein the ODL forms one of a pair of dielectric layers,
wherein the varying thicknesses provide different reflected colors
only visible with magnification of at least 10 times.
In a preferred embodiment the color difference between two covert
colors formed by a dielectric layer of different thickness as
described in this document has a .DELTA.E value of at least 10.
BRIEF DESCRIPTION OF THE DRAWINGS
Exemplary embodiments of the invention will now be described in
conjunction with the drawings in which:
FIG. 1 is a cross sectional view of multilayer Fabry-Perot foil in
accordance with an embodiment of the invention, wherein a variable
thickness stepped layer of organic dielectric material is shown
sandwiched between a uniform thickness reflector layer and a
uniform thickness absorber layer, wherein a square wave pattern is
embossed in the dielectric.
FIG. 2 is a cross sectional view of a symmetric multilayer
Fabry-Perot foil shows two similar structures shown back to back
sharing a common central reflector layer, in accordance with an
embodiment of the invention.
FIG. 3a, is a cross sectional view similar to FIG. 1, wherein a
release layer is provided between the structure shown in FIG. 1 and
a substrate, providing an embossed foil on a releasable
substrate.
FIG. 3b is a cross sectional view of a non-symmetric Fabry-Perot
chromagram having a dielectric spacer shown with two different
thickness and wherein the spacer is embossed with a grating.
FIG. 4 is a plan view of a single Fabry-Perot flake having a single
row and multi-column array of covert colored regions, wherein
adjacent colored regions display a different color.
FIG. 5 is a color chart showing the gamut of colors due to
embossing in a Fabry Perot cavity for an organic layer embossed to
different thicknesses.
FIG. 6 is a cross sectional view of a non-symmetric Fabry Perot
structure having color effects viewed from either side.
FIG. 7 is a diagram of an in-line vacuum roll coater for making
holograms in accordance with an embodiment of this invention.
FIG. 8 is a diagram of an in-line vacuum roll coater for making
demetallized Chromagrams.TM..
FIG. 9 is a diagram of an in-line vacuum roll coater for making
demetallized Chromagrams.TM..
FIG. 10 is a diagram of an embossing station wherein a plasma
treatment unit is provided to reduce the surface energy of the shim
to lessen sticking.
FIG. 11 is a diagram of an embossing station wherein a plasma
treatment unit is provided to provide UV cure of the polymer as it
leaves the shim.
FIG. 12 is a diagram of an embossing station combining the
embodiments shown in FIGS. 9 and 10.
FIG. 13 is a diagram of an embossing shim on an embossing roll
illustrating unwanted deflection of the impression roll.
FIG. 14 is a diagram of a preferred embodiment wherein little or no
depression of the impression roll.
FIG. 15 is a diagram of a polymer coating station wherein a train
of rollers is used to reduce the thickness of the coating
monomer.
FIG. 16 is a diagram of a polymer coating station wherein a train
of rollers is used to reduce the thickness of the coating monomer
similar to FIG. 15, wherein a heated roll is provided to vaporize
the monomer.
FIG. 17 is a diagram of a polymer coating station wherein a train
of rollers is used to reduce the thickness of the coating monomer
and wherein a slot die is provided to deposit monomer on the first
roller.
FIG. 18 is a cross section diagram illustrating a deposition drum
having various components in communication therewith.
FIG. 19 is a cross sectional side view of electron beams
penetrating a thin aluminium layer through and into the embossible
polymer layer.
DETAILED DESCRIPTION
Turning now to FIG. 1, a portion of a sheet of foil 100 is shown in
cross section, wherein the foil 100 includes a bottom reflector
layer 102 having a uniform thickness; upon the reflector layer 102
is deposited on an organic dielectric layer 104 which is embossed
so as to have a varying thickness and providing dielectric spacer
regions differing in thickness. An absorber layer 106 is deposited
to a uniform thickness over the variable thickness organic
dielectric layer 104. In a preferred embodiment, the size of the
adjacent regions (a) through (e) should be less than the size of a
pixel or element that can be seen by the human eye. However, the
invention does not require all adjacent steps or different
thickness regions to be less than the size a human eye can see,
however there must at least be one such element or region to
provide the covert desired feature. For example, any element (a)
through (e) could be sized to be small enough so that magnification
is required to see it, whereas adjacent elements can be large
enough to be seen by a human eye; however, preferably, several
adjacent pixels or pixels within a sheet or flake are of dimensions
that cannot be individually seen by a human eye. Furthermore,
preferably the several adjacent pixels under magnification show
distinctly different colors, thereby providing a covert color code
or pattern, hidden within the structure. With reference to FIG. 1,
if the totality of regions, a, b, c, d, and e comprise an area less
than 100 microns square, which is the approximately the smallest
region an unaided eye can see, distinguishing different colors from
a, b, and c will not be possible.
Since the dielectric layer in regions, a, b and c are purposefully
embossed with different thicknesses, using judicious selection of
the embossing depths, light reflecting back to the viewer after
impinging upon the reflector will be three different distinct
colors. However due to the small size of the regions a, b, and c,
the eye will tend to integrate and if the pixel or region defined
by (a) through (d) inclusive can be seen; only a single color will
be perceived. With sufficient magnification, the individual regions
(a), (b), and (c) will be seen and different colors will be
perceived.
It is preferred the color difference be significant enough to be
clearly identifiable, and not just distinguishable between two very
close colors.
In the La*b* color space system, the colors are plotted in a plane
of the CIELAB-system in which a* represents red and green and b*
represents yellow and blue. The lightness of the color is on an
axis at right angles to the plane going from black or L*=0 to white
where L*=100. Thus the color would be grey in the center of the
plane with the chroma increasing from the center toward the outer
perimeter of the plane. The extreme edge of the plane defines the
highest chroma. For example, a red light emitting laser would have
high chroma. Between the center and edge, there are various
gradations of the red as for example, a pink. Thus, there are
planes of these colors which move up and down the L* axis or the
lightness value axis. For every illuminant-observer combination of
the tristimulus value, the color coordinates can be readily
calculated and also can be measured. It is well known to those
skilled in the art of color, that any pigment, colored foil or any
color can have a different appearance depending upon the
illuminant. For example a color under fluorescent light may be
quite different from the color under sunlight or under a tungsten
lamp.
Thus a pigment may be irradiated with a predetermined amount of
energy across the wavelength to provide a graph of power versus
wavelength. The quantity of light or energy impinging or striking
the pigment at a given wavelength will influence the reflectance
curve. The spectral power distribution from the light source is
integrated with the eye response function typically designated as
x, y and z and the reflectance spectrum to yield the tristimulus
values X, Y and Z.
In connection with the present invention, the L*, a*, b* (CIELAB)
color space in used to describe the invention since this system is
the most uniform (linear in color) known to date and is generally
accepted worldwide for practical use. Thus, in the CIELAB color
space, the color of any optically variable device can be
characterized by the three tristimulus values, X, Y and Z. These
tristimulus values take into account the spectral distribution of
the light source, the reflectance of the optically variable pigment
and the spectral sensitivity to the human eye. It is from these X,
Y and Z values that the L*, a*, b* coordinates are calculated as
are the related values of L* (lightness), C* (chroma), h (hue) and
associated color differences i.e. delta L*, delta C* and delta h.
Thus any color can be represented by the L, a* b*. color (L.sub.1*,
a.sub.1*, b.sub.1*) and color (L.sub.2*, a.sub.2*, b.sub.2*) is
defined as:
The difference between any two colors,
.DELTA.E*.sub.ab=[(.DELTA.L*).sup.2+(.DELTA.a*).sup.2+(.DELTA.b*).sup.2].-
sup.1/2
In effect, a covert color code not recognizable by an unaided human
eye is present within the coating, wherein only a single integrated
color is perceived by an unaided eye looking at the structure. The
thickness of the dielectric in these covert different color regions
differs and is uniform throughout a region. Preferably, these
regions form a square or rectangular wave pattern, however this
pattern need not be periodic.
Similarly a color shifting dielectric stack of high and low index
dielectric layers can serve as a covert coating by using one or
more dielectric layers wherein the thickness varies similarly, such
that at least one region having a thickness distinct from other
regions, is not visible by the unaided human eye, but is
distinguishable with suitable magnification.
One way in which to manufacture the structure of FIG. 1 is to (a)
provide an incoming roll of polyester; (b) coat one side with an
absorber such as Cr; (c) evaporate the organic dielectric layer
over the Cr layer; (d) emboss the organic dielectric layer; and,
(e) coat the embossed organic dielectric layer with a reflective
layer with a material such Al.
Referring now to FIG. 2 a symmetric structure is shown having small
regions of different thicknesses. The structure depicted in FIG. 2
is similar in many respects to that described above in FIG. 1,
however, the reflector layer is a common layer to two same embossed
dielectric, absorber coated layers. Conveniently, the structure
shown in FIG. 2 lends itself to being broken or ground into pigment
flakes or particles. Notwithstanding, the structure shown in FIG. 1
can also be used as non-symmetric pigment coating.
Although the cross section shown in FIGS. 1 and 2 would lead one to
conclude the regions are square or rectangular, this invention is
not limited to embossing only squares, rectangles, and the like.
Circles, triangles or other regions can be embossed, as long as
some of the regions are small enough not to be detected without
significant magnification. The structure shown in FIG. 2 is made by
passing an aluminum foil that had been coated on both sides with
the organic spacer 104 through opposing embossing rollers. The
organic layer may be deposited by passing the aluminum foil through
a bath by moving the foil through a bath containing an organic
coating/solvent and pulling the aluminum foil out straight out
using a process known as dip-coating, followed by drying off the
solvent. Alternatively the organic coating 104 could be applied on
one side, cured, and after flipping over the roll, the other side
is coated. After embossing the organic coating the absorber layer
is deposited onto the embossed organic layers. Alternatively, a PET
substrate coated symmetrically on both sides with a reflector layer
and an organic spacer is passed between the opposing embossing
rollers and then the absorber layer would be deposited onto each of
the embossed layers. The absorber is subsequently added to each
side and then embossed. The absorber may be an electro-less deposit
of metal such as nickel, silver or tin or could be any
semi-transmissive layer deposited from vacuum. The two-sided
embossed structures provide a material suitable for chopping into
glitter which would be a unique color shift material. Depending the
desired result both organic layers need not be embossed. Although
it is preferably that the embossed regions are flat, and
substantially parallel to the reflector layer, this need not be and
the layer may be angled at an angle other than zero with respect to
the reflector layer. In a preferred embodiment the covert color
pattern within a covert sheet or flake has an embossed dielectric
with at least three distinct thicknesses and at least three
distinct covert colors.
Turning now to FIG. 3a a structure similar to FIG. 1 is shown
wherein the embossed organic structure is shown upon a substrate
132 having a release layer 130. Once the foil is stripped from the
substrate it can be used to make flakes, which have covert features
therein.
A hot-stamped structure can be made very similarly to the structure
of FIG. 1, but in reverse order, wherein the steps employed are:
providing a polymer release layer on a substrate; depositing a
chrome layer; evaporating an organic dielectric layer; embossing
the organic dielectric; depositing a reflective layer, followed by
an adhesive layer.
Referring to FIG. 3b, a different embodiment of a Fabry-Perot
diffractive structure is shown, having a dielectric layer 104
embossed to one thickness as shown with the dotted line. If one
embosses into the dielectric layer of a Fabry Perot filter the
average depth of the embossing to a first approximation is the
midpoint between the average ridge height and the average valley
depth. The region shown in dotted outline is not flat and has
ridges and valleys therewithin. Since the distance between ridges
and valleys are below the resolution one one's eye, all one sees
from the Fabry Perot filter after the Cr absorber layer 106 is
added to the dielectric layer is a color shift underneath the
relief structure that is a different color shift from those areas
not embossed. The colors arising from the different thicknesses of
the ridges and valleys blend together so as to appear to the eye as
being silver. Thus, the combination of the color from the embossed
area is the sum of the effects of the ridges and valleys of the
dielectric, that is, the silver-like color plus the color shift
from the apparent uniform thickness of the underlying dielectric
plus the diffractive effect.
FIG. 4 is an illustration of a Fabry-Perot flake 180 having lateral
dimensions of about 17 microns and wherein an array or pattern 190
of 2 micron embossing of squares having a different uniform depth
are shown varying from blue to green in color effect. However due
to the size of the 2 micron squares, magnification of at least 50
times is required to see the color coded squares and to discern
their colors, and preferably, a 400 times magnification is required
to comfortably distinguish the color of the 2 micron squares. If
the pixel element was 80 microns square a magnification of about
1.25 would be required to just see it, and to see it would ease, a
magnification of approximately 12.5 would be required.
FIG. 5 is an illustration showing the gamut of colors due to
embossing in a Fabry-Perot cavity for an organic layer embossed to
0.232-0.442 microns.
An alternate structure is illustrated in FIG. 6 that has similar
but different effects from the symmetrical structure shown in FIG.
3. Although the reflector layer 102 is shared between two Fabry
Perot cavities, the upper cavity has the covert coating therein,
wherein the lower cavity displays a single color with no covert
features therein. The lower cavity dielectric layer 104b and 106b
could also be made having a thickness that would provide a color
effect which was similar in appearance to the color that is
integrated by the brain after viewing the variable thickness
Fabry-Perot structure. Thus, if flakes were made, and were small
enough, the perceived color would be essentially uniform with
little, if any, perceived variation. Although embodiments shown
heretofore illustrate several different thicknesses within a flake
or regions, it would be possible to make a flake having a
dielectric layer with only a single small region with a different
thickness than the rest of the flake to provide the covert feature.
It should also be noted that the covert region having the different
thickness dielectric can be in the form of a logo or other indicia
that can easily be recognized when sufficiently magnified.
Turning now to FIG. 7 an in-line system is shown for making a
holographic relief structure. The process flow starts by
introducing a web of plastic into the vacuum chamber at the unwind
station 80. The plastic web may have a coating of a release
hard-coat layer for hot stamping the final product, or
alternatively, the plastic web may be an uncoated plastic film such
as a polyester terephthlate (PET) film for making labels or
security threads. On top of the plastic web or release hard-coat
layer is a resin layer capable of being embossed. A single layer
employing a release hard coat that is embossible may also be used.
The web then moves to the first station, Chamber 5, and is coated
uniformly with aluminum, after which the coated web is embossed in
Chamber 1. This produces a reflective, relief type structure, by
way of example, holograms capable of being hot stamped onto
articles for security protection or feed for pressure sensitive
labels.
FIG. 8 shows a complete process for making a demetalized
Chromagram.TM. in one or two passes. Each chamber is situated in
its own pumping chamber (not shown) and each chamber is used as a
module for each separate operation. By using this type of modular
approach, each module can be physically moved and interchanged for
another module within the vacuum machine so that the order of
operations can easily changed for variations in the way the
security products are made.
The processing chamber includes an unwind reel 80, Chamber 1 which
has an embossing roller 82; a registration sensor 83 is provided
between Chamber 1 and 2. Chamber 2 includes of an oil patterning
unit that includes an oil pick-up roller 84, an oil-patterning
roller 85 and a resistance source of Al 86 and an optional UV or
electron beam cure station. A plasma treatment unit 97 comprising
an O.sub.2 plasma source is included after the aluminum deposition
but before the first front surface roll to ensure that any residual
oil is burned off and does not contaminate the metallized surface
or give ghosting. Chamber 3 has of an array of DC magnetron
sputtering units 87 for depositing the absorber layer. Chamber 4
includes a processing unit to deposit organic acrylics followed by
a UV cure station 89. Chamber 5 has a multi-pocket crucible 90 for
e-beam coating by electron beam gun 91 of either an inorganic
dielectric or a reflective metal. Transmittance monitors 92, 93 and
95, a reflection monitor 94 are also provided within the modular
vacuum roll coater system.
A variation of this process described above allows for the
production of non-demetallized (non-demet) Chromagrams.TM.. In this
process flow, the resin layer on the incoming web is embossed in
Chamber 1, then coated with an absorber layer at Chamber 3 and is
subsequently coated in Chamber 5 with an inorganic dielectric such
as MgF2. The roll is then reversed and coated in Chamber 2 with a
reflector with the patterning unit turned off so as to complete the
Fabry Perot structure. Finally, the non de-met Chromagram.TM. is
wound onto the unwind roller 80. This process flow is shown in the
second flow process in FIG. 8.
De-met Chromagrams can also be produced using color shifting ink or
adhesive.
Another variation of the processes described above, wherein the
same modular system can be used is where the PET release hard coat
encounters Chamber 1 and 2 to produce demetallized holograms. After
passing Chamber 2 the roll of PET including de-met holograms is
wound directly onto the wind-up roll 86. This can then be
subsequently processed off-line with color shifting ink or color
shifting adhesive to make hot stamp Chromagrams.
Referring now to FIG. 9 an in-line vacuum roll coater system is
shown for making Demet Chromagrams.TM.. In this embodiment, an
embossing resin is applied as a first step. A PET web is introduced
to the vacuum coater and coated with the acrylic coating in Chamber
4. In this instance, the lacquer is a coating of a UV curable
acrylic monomer based on a technology available from Sigma
Technologies Inc of Phoenix, Ariz. A UV lamp or e-beam is provided
at Chamber 4 to provide partial or full cure of the acrylic layer
takes place at curing station 9 depending on the monomer used. If a
partial cure is used, then a full cure by another UV source or
electron beam 96 is used following the aluminum deposition by UV
curing using a UV lamp 9 transparent substrate or e-beam through
the non-UV transparent web following Chamber 4.
Since polymers are degraded by UV light, an alternative embodiment
is provided, in order to lessen the likelihood of degradation and
decay of the polymer substrate due to UV absorption in accordance
with Beer's Law. Therefore in this embodiment completion of the
polymer is achieved by the use of an electron beam gun but
positioned of the metal side of the substrate before the first
front surface roller after the aluminum deposition rather than via
a UV lamp. In this instance the electrons penetrate the aluminum
coating. It could be expected that the aluminum thickness would be
in the range 10-100 nm and penetration is sufficient using electron
beam curing in air where the electrons have to penetrate a titanium
foil of the order 7 microns thickness. Preferably, the electron
beam source be driven at a higher voltage in order to penetrate to
the full polymer depth than if there were no metal there at all but
this would still be much less than an atmospheric electron beam
cure system. Although not shown in the figure, plasma treatment
following the polymer coating may be provided to increase the
surface energy to improve the metal adhesion.
Referring now to FIG. 18 a system is shown wherein a reel 182a
provides an uncoated web 183 of substrate material to a deposition
drum 180 and further to a subsequent reel 182b. After leaving the
reel 182a the web 183 passes through the deposition unit 185 and
the plasma treater 184 for a partial cure of the deposited polymer.
A reflective metal such as Al is deposited on the organic
dielectric coated web 183 via a metal deposition source 186. An
e-beam gun 181 penetrates the thin Al coating 192 to fully cure the
embossible polymer 194. FIG. 19 is a cross-sectional view of the
e-beams 193 penetrating the thin Al layer 192 to fully cure the
embossible polymer 194 layer beneath supported by the polymer
substrate 183.
Furthermore, plasma treatments may be provided after metallization
before further polymer coatings are applied. Some of the polymers
do not adhere easily and so it is always preferable to prepare the
surface using plasma treatment to make sure the surface energy is
maximized to help the depositing coating wet out the surface. The
plasma treatment before applying the polymer was an argon/nitrogen
plasma treatment so that there was no oxygen present to inhibit the
polymer cure whereas the plasma treatment of the acrylate before
the metal deposition was using an argon/oxygen plasma to provide
some oxygen to aid chemical bonding directly between the polymer
and metal via the oxide
After the web passes from Chamber 4 it then encounters Chamber 2
where a patterned or non-patterned aluminum layer is deposited. A
plasma treatment O.sub.2 source 97 is provided to clean up any
residual oil and to prevent or lessen ghosting. The web then moves
to Chamber 1.
Preferably, Chamber 1 uses a drum that is compliant so that the
embossing is effective. The steel drum can be wrapped with a hard
rubber sleeve that does not outgass. The embossing roller can be
heated and the rubber on the drum cooled. Embossing can be
selectively applied or over the entire surface of the web or can be
applied as indicated just on the aluminum islands. The oil
patterning process technology is commercially available. At this
point, the roll can travel directly to the wind-up roller without
the use of other process steps for the production of demetallized
holograms or other types of de-met relief structures or non
demetallized structures. These structures can then be subsequently
coated with color shift ink in the non-aluminized areas to make ink
based or OVP adhesive based Chromagrams.TM.. This coating
arrangement allows for embossing into the aluminum which eliminates
the possibility of the organic layer fouling of the embossing
roller.
For further processing, the web travels to Chamber 3 and 5 where it
is coated with the absorber layer (Cr), a dielectric layer
(MgF.sub.2) and a full reflective layer (Al) is deposited in the
reverse direction. In this instance, final wind-up occurs on the
un-wind roller 81. Alternatively, the web can be coated with
absorber layer, reversed in the machine and sequentially coated
with the organic acrylic layer followed by the reflective aluminum
layer to make a Chromagram with an organic dielectric rather than
an inorganic dielectric. In this case, also the final wind up
occurs at the un-wind station.
Process flows include:
1) Plastic film (e.g. PET type G)embossaluminize across whole width
of webhologram or diffractive label.
2) Plastic film plus release/resin)embossaluminize across whole
width of webhot stamp relief reflective hologram or production of
diffractive flake with/without symbols.
3) Plastic filmUV acryliccurealuminizeembosshologram or diffractive
label.
4) Plastic filmUV acrylicpartial curealuminize in patternembossfull
cureEvaporation of absorber layerdeposition of dielectric
layerdeposition of reflector layerDe-met Label Chromagram. (Note:
embossing may be in register with the aluminum or across aluminum
and non-aluminum areas.
5) Plastic film with release/resinaluminize in
patternembossDeposition of absorber layerDeposition of dielectric
layerdeposition of reflector layerHot stamp demet Chromagram.
6) Plastic film with release/resinDeposition of absorberUV
acrylicDeposition of reflectorDeposition of reflectorHot Stamp
Non-Demet Chromagram.
7) Plastic filmDeposition of absorberUV acrylicDeposition of
reflectorDeposition of reflectorLabel Non-Demet Chromagram.
Embossing
Embossing can be done in a variety of different ways. The shim can
be pressed into the polymer with enough pressure to force the
polymer to flow into the shim profile. This becomes easier if heat
is used to soften the polymer. Alternatively lower melting point
polymers can be used or even in the extreme case a liquid monomer
can be used where the cure is done, whilst the liquid has taken up
the shim profile and solidified the polymer.
When using pressure, or heat and pressure, there is a tendency for
the polymer to relax slightly following easing the pressure and the
polymer partly recovers the flat surface. Thus the diffraction
grating or hologram can appear less bright that the shim original.
Full depth embossing usually requires some heat and pressure used
together; immediately following the hot nip there is a chilled roll
to remove the heat as fast as possible to limit the amount of
relaxation of the polymer.
The UV cure process that cures the polymer whilst still in contact
with shim provides the best chance of obtaining a full depth
embossing however some release difficulties can occur.
Embossing can be difficult even at atmospheric pressure, and the
degree of difficulty can depend on the quality of the embossing
shims and on the profile of the embossing pattern. For example
sinusoidal and pyramidal patterns are easier profiles to work with
compared with square wave zero order diffraction type or deep
aspect ratio patterns.
The problem is for the shim to release the malleable polymer. At
times the polymer may separate from the substrate and clog up the
shim, which then fails to emboss on the next and subsequent
revolution. To minimize the propensity for the polymer sticking an
operator, in a system at atmospheric pressures, would spray the
shim with a release agent.
Notwithstanding this problem is exacerbated in a vacuum system
where it becomes difficult to identify when the shim needs another
application of the release spray, and so a method of controlling
the shim surface is an aspect of this invention. In accordance with
an embodiment of this invention, and as is shown in FIG. 10, the
shim 101 can be treated with a contained plasma of a gas mixture of
argon and a second component such as fluorinated gas such that the
surface energy of the shim can be reduced and so that the shim
would be coated and surface treated to provide a low energy surface
analogous to a non-stick surface such as PTFE.
In FIG. 10 a plasma treater 104a is shown adjacent the embossing
roll 101 to fluorinate the shim as a method of providing a release
coating to the shim Using a fluorinated plasma with a low level of
fluorination would deposit a monolayer, or less, of a PTFE type
non-stick coating to the surface. Any monolayer or so that is lost
by being taken away by the polymer as it is released it would be
replaced during the next revolution of the shim past the
plasma.
Alternatively an inert gas plasma as a source of UV can be used to
sure cure the polymer in-situ whilst still in the shim. This would
not work so well if the polymer coating were already
metallized.
These could be combined in single plasma but the polymer surface
would then also become fluorinated and so non-stick. This would
make the polymer harder to add coatings to and so would really only
be useful if the polymer coating were to be the final layer.
Referring now to FIG. 11, Inert gas plasma is shown to provide UV
cure of polymer as it leaves the shim to help maximise the
embossing depth. Also if the UV penetrates to cure the polymer
whilst it is still in the shim it can aid the polymer release from
the shim in two ways. One is the polymer strength is increased by
the fuller cure and second as the cure takes place there is usually
a shrinkage in the polymer thickness which would help pull the
polymer out of the shim.
FIG. 12 illustrates a preferred embodiment where the use of two
plasmas 104a, 104b on the same shim 101 is provided. Alternatively
and less preferably would be to incorporate the two processes into
one plasma. By so doing, unfortunately the embossed polymer would
also have the low energy surface and this would make sticking the
next layer onto this surface more difficult.
Turning now to FIG. 13, an embossing setup is shown wherein a shim
130 on a non-compliant embossing roll is shown. When embossing
without a compliant roll it is common, with increasing pressure, to
cause a deflection of the impression roll 132 as is shown in dotted
outline, thus causing variations in the embossing depth from the
centre of the web to the edges.
An option shown in FIG. 14 is to use a hard impression roll 142
having stepped ends 144 that are beyond the edges of the shim, and
are larger in diameter. This defines the limit of how close the
rolls can be run together thus preventing the impression roll
deflecting as the load is taken up by the larger diameter ends of
the roll. The diameter of these ends needs to take account of the
substrate thickness plus the polymer layer that is to be embossed.
Advantageously, this allows either a harder compound compliant roll
or a hard metal impression roll to be used.
Polymer Deposition Process
Referring now to FIG. 15, a series of rolls 151a through 151f that
may be of different diameters and/or rotating at different speeds
are utilized as a method of reducing the monomer loading on
successive rolls to deposit a preferred amount of the monomer onto
the web 155 supported by a cooled deposition drum 157. Using
chilled rolls 151a through 151f would allow the use of some
polymers with vapour pressures that might otherwise be too high.
Alternatively using the printing style roller train as the means of
taking the monomer from the bath of liquid and reducing the monomer
thickness on each roll successively down to the desired
thinness.
FIG. 16 shows an alternative embodiment to FIG. 15 wherein a last
roll 151g is a heated roll that vaporizes the monomer. The vapor is
then condensed onto the substrate passing by on the cooled
deposition drum.
FIG. 17 illustrates a third variation wherein the monomer is
evaporated via a slot die coupled to the roll train to improve the
uniformity before coating the web either directly or via the hot
roll vaporization method.
Of course numerous other embodiments may be envisaged without
departing from the spirit and scope of the invention.
* * * * *
References